Method and apparatus for improving the grinding result of a pressure chamber grinder

ABSTRACT

The invention concerns a method and an apparatus for improving the grinding result of a pressure chamber grinder. According to the method the finely divided material to be ground is fed by means of a mechanical feeder device (1) into a pressurized equalizing tank (2), the possibly clodded material is made loose by means of a rotor in the equalizing tank, and the material thus made loose is transferred into a pre-grinder (3), wherein several grinding-gas jets are applied to the material to be ground so that the material to be ground is fluidized, the fluidized material-gas flow is passed into a bisecting device (6), wherein it is divided into two component flows of equivalent magnitude and composition, each component flow is passed into the main grinding chamber (9) through a long accelerating nozzle (8) of its own, which said nozzle is directed so that a collision zone for the two component flows is formed in the center point of the said main grinding chamber. The method is characterized by that a solids-gas mixture ground in the main grinding chamber (9) is passed through an acceleration tube (10) into a mechanical grinder (11) in a direction corresponding to the rotation direction of the grinder rotor (13) driven by an electric motor (12), whereby pivotably mounted grinding hammers of the grinder are arranged to break up the coarser particles, moved to the outer periphery of the grinder, before their exit through a central out-flow (15) of the grinder.

The present invention is concerned with a method and an apparatus forimproving the grinding result of a pressure chamber grinder. In themethod finely divided material to be ground is fed by means of amechanical feeder device into a pressurized equalizing tank, in theequalizing tank the possibly clodded material is made loose by means ofa rotor, and the material thus made loose is transferred into apre-grinder, wherein several grinding-gas jets are applied to thematerial to be ground so that the material to be ground is fluidized,the fluidized material-gas flow is passed into a bisecting device,wherein it is divided into two component flows of equivalent magnitudeand composition, each component flow is passed into the main grindingchamber through a long accelerating nozzle of its own, which said nozzleis directed so that a collision zone for the two component flows isformed in the centre point of the said main grinding chamber.

It is an advantage of such a pressure chamber grinder that, as regardsits energy economy, it is by far superior to conventional jet grinders,wherein ejectors are usually used as the feeder or accelerating device.Since in principle the material particles to be ground are subjected tothe grinding effect only once, it is necessary to resort to a separateclassifier in which the coarser particles are separated from thematerial-gas flow and returned, in one way or another, into the maingrinding chamber for regrinding. In practice this mode of operation isusable when in the ground material the fraction having the finalparticle size is relatively small, and the unground material to bereturned to the grinding, is high.

If instead the material fraction returned to the grinding is relativelysmall it is questionable whether the use of a separate classifier ismotivated. The Finnish patent application No. 854671 discloses asolution for this kind of cases, according to which a pressure chambergrinder and a so called free-flow grinder a coupled in series. Thissystem has proven to be especially suitable when the material ground inthe pressure chamber grinder contains a very small fraction of ungroundmaterial and when the material is of a very small particle size.

In practice it has been noticed that all such systems based on grindingthe material solely using jet grinding techniques suffer from relativelyhigh operating costs. Especially the grinding of a material containingafter the pressure chamber grinder more than about 50% of a product ofthe final particle size involves unnecessary high costs if thepreviously mentioned equipment is applied. Among such materials can benamed, for instance, paper fillers, such as talc, as well as variousfoodstuffs, such as for instance corn and cocoa.

The object of the present invention is to eliminate these problems. Thishas been achieved by means of a method which is characterized in thatthe material-gas mixture ground in the main grinding chamber is passedthrough an acceleration tube into a mechanical grinder in a directioncorresponding to the rotation direction of the grinder rotor driven byan electric motor, whereby the pivotably mounted grinding hammers of thegrinder are arranged to break up coarser particles, moved to the outerperiphery of the grinder, before their exit through a central outflow ofthe grinder.

By using such a solution, the desired final result is obtained without aseparate classifier or a secondary grinder of free-flow type, and inaddition with essentially better energy economy. In the mechanicalgrinder the grinding conditions are chosen so that only the oversizeparticles are ground and the finer particles pass through thissubsequent grinder almost without delay. The method and apparatusaccording to the invention is especially advantageous when an unusuallyhigh degree of fineness in the final product is not required and whenthe hardness of the material to be ground is not high. Thus, especiallysoft minerals and foodstuffs are suitable.

In the following, we will describe the invention in more detail withreference to the attached drawing, wherein

FIG. 1 is a schematical illustration of the particle size distributionof the final product when a pressure chamber grinder alone is used, aswell as when an embodiment in accordance with the present invention isused,

FIG. 2 is a side view of an exemplifying embodiment of the apparatus ofthe present invention, and

FIG. 3 is a top view of the apparatus, partly in section.

The apparatus in accordance with the invention comprises a mechanicalfeeder 1, which may be either a plug feeder, by means of which finelydivided material to be ground is fed into a pressurized equalizing tank2 as a gas-tight plug by means of a push piston, as is described in theFinnish patent application No. 84 4264, or a valve feeder, as isillustrated in FIGS. 2 and 3. The use of such a valve feeder isdescribed, e.g. in the Finnish patent application No. 84 4028, so thatits operation will not be described in further detail in thisconnection. The possibly clodded material is made loose by means of arotor (not shown) in the equalizing tank and is transferred at a presetrate into a pre-grinder 3 by means of a screw conveyor 4. In theequalizing tank 2, approximately equal pressure is maintained ascompared with the pre-grinder 3. In the pre-grinder 3, several stronggrinding-gas jets are applied to the material to be ground, so that thematerial to be ground is fluidized. Grinding gas is passed into thepre-grinder through a gas pipe 5.

The fluidized material-gas mixture is made to rush from the pre-grinder3 into a bisecting device 6, where the said material-gas jet is dividedinto two component flows of equivalent magnitude and composition. Thetwo outlet pipes 7 of the bisecting device 6 are connected to the twolong accelerating nozzles 8 of the pressure chamber grinder, which saidnozzles are preferably shaped like venturi tubes. The acceleratingnozzles 8 are directed so that the component flows rushing through themat an increasing velocity collide with each other in a collision zoneformed in the middle point of the main grinding chamber 9. A highlyefficient grinding of the material particles takes place in thiscollision zone. If, by chance, the coarsest particles in thematerial-gas mixture collide in the main grinding chamber 9 only againstparticles of a considerably smaller size, the grinding remainsincomplete in respect of these coarser particles.

When the material-gas flow coming from the main grinding chamber 9 ispassed through the accelerating tube 10 into the mechanical grinder 11at a high velocity the material-gas mixture is forced into a rapidcirculatory movement so that, by the effect of the centrifugal force,the coarsest particles remain in this grinder 11 longer and becomeground by means of the grinding hammers 14 mounted on the rotor 13 androtating at a high velocity, whereafter the ground particles escapethrough a centrally placed exhaust pipe 15.

The rotor 13 is driven by an electric motor.

The grinding conditions should preferably be chosen so that only theexcessively large particles become ground in the mechanical grinder 11.By adjusting the grinding pressures so that a positive pressure of about0.1 to 1.0 bar prevails in the main grinding chamber, the incomingvelocity of the material-gas flow can be chosen to be suitably for theoperation of the mechanical grinder.

The rotor 13 of the mechanical grinder 11 comprises advantageously twodisc-like plates mounted at a mutual distance on the drive shaft,between which plates the grinding hammers 14 are tiltably mounted onpivot shafts located along the outer peripheries of the plates. Theinlet orifice of the out-flow pipe 15 is thereby centrally placedbetween the rotor plates. By this arrangement it is guaranted that allmaterial particles flowing into the mechanical grinder 11 are forced toflow through the operation zone of the grinding hammers 14.

The rotor 13 may also be of a multiple construction, whereby the evenlydistributed grinding hammers 14 are situated in these layers, one placedupon the other, and between each layer is placed a disc-likeintermediate plate. This construction improves markably the grindingcapacity of the mechanical grinder.

In order to guarantee the best possible flow conditions the inletorifice of the mechanical grinder is of a smaller diameter than its exitorifice.

In order to improve the capacity of the pressure chamber grinder theouter surface of the rotor 13 top-disc may be furnished with essentiallyradial flanges, which accomplish a fan-effect in the mechanical grinder11.

On the accelerating tube 10, the shape of which is preferably that of aventuri tube, a manometer may be installed in order to permitobservation of the pressure prevailing in the tube 10.

From the graph of FIG. 1 it is clearly seen that the particledistribution obtained by means of a solution in accordance with thepresent invention is much steeper than that obtained using a pressurechamber grinder alone. The vertical parameter is the percentage ofpenetration of the final product, and the horizontal parameter is theparticle size. Since both curves intersect each other at a penetrationvalue of 50%, the average particle size obtained with both of themethods is the same.

What is claimed is:
 1. Method for improving the grinding result of apressure chamber grinder, wherein the finely divided material to beground is fed by means of a mechanical feeder device (1) into apressurized equalizing tank (2), the possibly clodded material is madeloose by means of a rotor in the equalizing tank, and the material thusmade loose is transferred into a pre-grinder (3), wherein severalgrinding-gas jets are applied to the material to be ground so that thematerial to be ground is fluidized, the fluidized material-gas flow ispassed into a bisecting device (6), wherein it is divided into twocomponent flows of equivalent magnitude and composition, each componentflow is passed into the main grinding chamber (9) through a longaccelerating nozzle (8) of its own, which said nozzle is directed sothat a collision zone for the two component flows is formed in thecentre point of the said main grinding chamber, characterized in that asolids-gas mixture ground in the main grinding chamber (9) is passedthrough an acceleration tube (10) into a rotary mechanical grinder (11)in a direction corresponding to the rotation direction of the grinderrotor (13) driven by an electric motor (12), whereby pivotably mountedgrinding hammers of the grinder are arranged to break up the coarserparticles, moved to the outer periphery of the grinder, before theirexit through a central out-flow (15) of the grinder.
 2. Method asclaimed in claim 1, characterized in that the grinding pressures areadjusted so that a positive pressure of about 0.1-1.0 prevails in themain grinding chamber (9) enabling the velocity of the material - gasflow to the mechanical grinder to be selected so that only oversizeparticles are ground in the mechanical grinder.
 3. Apparatus forimproving the grinding result of a pressure chamber grinder, which saidapparatus comprises a mechanical feeder device (1), a pressurizedequalizing tank (2) jointly operative with the feeder, which saidequalizing tank is provided with a rotor and with a screw conveyor (4)for carrying the material to be ground into a pre-grinder (3), intowhich the grinding gas is passed through a gas pipe (5), a bisectingdevice (6) provided at the outlet side of the pre-grinder (3), both ofwhose outlet pipes (7) are connected to long accelerating nozzles (8) oftheir own, terminating in the main grinding chamber (9) and beingdirected so that the material-gas jets rushing out of them collideagainst each other int eh centre point of the grinding chamber (9),characterized in that to the outlet end of the main grinding chamber(9), via an acceleration tube (10), a mechanical grinder (11) isconnected, comprising a rotor (13) furnished with pivoted grindinghammers (14), in which grinder (11) the acceleration tube (10)terminates essentially tangentially, in the rotational direction of therotor, and at the centre of which grinder an out-flow pipe (15) isprovided for the ground final product.
 4. Apparatus as claimed in claim3, characterized in that the rotor (13) comprises two disc-like platesmounted at a mutual distance on the drive shaft, between which platesthe grinding hammers (14) are tiltably mounted on pivot shafts locatedalong the outer periferies of the plates, and that the inlet orifice ofthe out-flow pipe (15) is centrally placed between these two plates. 5.Apparatus as claimed in claim 3, characterized in that the accelerationtube (10) has a shape of venturi tube and a manometer is provided inorder to observe of the pressure in the tube (10).